A ground anchor method (earth anchor or ground anchor) refers to a landslide protection method, a slope reinforcement method, etc., for perforating the ground, installing a tensile material included in an anchor body, injecting and hardening grout to ensure bonding force, and applying tension to the tensile material included in the anchor body to counter ground pressure. After first attempted in 1874, a ground anchor method is recognized as an excellent method and has been widely used in various fields such as deep excavation, revetment reinforcement, water pressure counter, and tunnel and cut slope stabilization in civil engineering and construction sites and related technologies have been continuously developed.
A ground anchor has a bond length part that exerts stress of an anchor, a free length part that transfers the stress exerted by the bond length part, and an anchorage zone part that is an element for transferring consumed stress of the anchor directly to a structure. The bond length part is designed to sufficiently exert consumed stress in consideration of a ground type, a bonding method, a type of a tensile material, a grouting method, and so on and it is required to select appropriate bond stress τ according to a ground type. Except for a specific case, it is well known that an anchor becomes mostly damaged in the free length part and, thus, a function of the free length part is important. The free length part is formed of a steel bar, a steel strand, a steel wire, or the like and, in this regard, appropriate corrosion protection needs to be processed on the free length part according to purpose of use and a required persisting period. The anchorage zone part is classified into a button-type part, a wedge-type part, a nut-type part, and so on and its type is determined in consideration of future maintenance, purpose of use, and so on. In terms of a period of use, a ground anchor is classified into a temporary construction anchor that is removed after being used for a period of less than two years and a permanent anchor that is permanently applied along with a structure.
A long-term stress loss phenomenon inevitably occurs over time in a ground anchor due to creep characteristics, etc., of the ground. The phenomenon is based on a behavior and displacement of the ground around a bond length part of a ground anchor that is positioned in the ground and exerts bonding force of the ground anchor, and it is very difficult to accurately predict and check a behavior of the ground around the bond length part.
A technology known as a method of checking existing stress of a ground anchor includes a method of installing a load cell on each anchor, a method of checking existing stress via a lift off test, and the like. The method of installing a load cell requires installation costs of a load cell, power for measurement, a data logger, and so on and, thus, much costs for measurement are required and it is difficult to accurately determine a time point for measurement. In addition, for long-term measurement, a load cell is installed outdoor and, thus, measurement accuracy is disadvantageously degraded due to degraded durability.
The method of checking existing stress of a ground anchor via a lift off test also requires excessive test costs because of test equipment, test manpower, and so on and, thus, has not been widely used in reality. For the reasons thereof, a conventional method of checking existing stress of a ground anchor selectively has examined only some samples instead of all installed ground anchors in accordance with current trends.